1. Field of the Invention
The invention relates to a power transmission mechanism for optical system of image forming apparatus, and more particularly to a power transmission mechanism for creating images by varying a magnification factor of an original image.
2. Description of the Related Art
In this kind of optical system, generally, as a moving frame for illuminating and scanning the original, a first moving frame possessing a first mirror for guiding the fluorescent lamp for illuminating the original or the reflected light from the original to a specified optical path, and a second moving frame fixing second and third mirrors are disposed so as to be movable reciprocally in a state parallel to the original. By setting the moving speed of the first moving frame at a specific rate (1/2 speed in the case of equal magnification) to the moving speed of the second moving frame, the original is illuminated and scanned (see, for example, the unexamined Japanese Patent Publication No. 315775/1989).
On the other hand, in the optical path from the third mirror of the second moving frame to a fourth mirror fixed on a third moving frame for adjusting the length of optical path, lenses of a lens unit are disposed. The lens unit is composed so as to be free to displace relatively, in the state along the optical path, with respect to the second moving frame, and by setting the optical length from the lens to the image creating area at a specific rate (1/2 in the case of equal magnification), for the optical path length from the original to the lens, the original image can be focused on the image creating area at a desired magnification factor.
To displace the lens unit relatively, a driving wire is fixed in the lens unit, and it is constituted to drive this driving wire in a specified direction by an optical system motor. On the other hand, the lens unit and the third moving frame are coupled through a link mechanism, and by interlocking the third moving frame in response to the displacement of the lens unit, the optical length from the lens to the image creating area was adjusted.
Specifically, the driving wire for driving the lens unit was wound around a cam pulley, and a part was fixed on the third moving frame. The cam pulley was eccentric to the center of outer circumference of winding the driving wire in the center of rotation, in order to displace the third moving frame to the corresponding position along the displacement of the lens unit.
To distribute the image forming apparatus comprising such optical system widely for household and personal use, it is desired to reduce the size and weight as much as possible. In the link mechanism of such conventional optical system, since the center of rotation is the cam pulley eccentric to the center of the outer circumference of winding the driving wire, it is necessary to set the outer diameter and the eccentricity of the cam pulley considerably larger, and the lens unit itself becomes larger in size, the reduction of size and weight of the apparatus cannot be achieved. In particular, to set the variable range of magnification factor wider, the interlocking range of the third moving frame must be set larger, which adds to the inclination of increase in size. This fact substantially limited the range of enlarging value and reducing value of the original image in the conventional image forming apparatus developed for household and personal use.
In the light of such inconvenience, there arises a true necessity of power transmission mechanism of optical system capable of setting wider the range of enlarging value and reducing value of the original image of the image forming apparatus developed for household and personal use, not alone downsizing the lens unit itself and achieving the reduction of size and weight of the entire apparatus.
To illuminate and scan the original, the first and second moving frames were coupled to a reciprocal drive mechanism for transmitting a torque from a torque source of the optical system drive unit.
On the other hand, to vary the magnification of the original image, the lens unit was coupled to a magnification varying drive mechanism. The magnification varying drive mechanism partly received the torque from the torque source of the optical system drive unit through constituent parts of the reciprocal drive mechanism. Accordingly, in the magnification varying action of the lens unit, when the torque is transmitted from the torque source of the optical system drive unit to the magnification varying drive mechanism, the first and second moving frames follow the lens unit, and move from the home position.
Besides, transmission of torque to each drive mechanism was effected through gear engagement. In the conventional constitution, hence, it was necessary to control positioning of the output member of the torque source and the input member of the drive mechanism in the rotating direction.
In the torque transmission mechanism of the conventional optical system, in this way, in the magnification varying action of the lens unit, it was constituted so that the first and second moving frames would follow the lens unit and move from the home position, and therefore when illuminating and scanning the original after varying the magnification factor of the original image, it was necessary to return each movable frame to the home position, and the control was complicated. In the conventional constitution, furthermore, it was necessary to control the positioning between the output member of the torque source and the input member of the drive mechanism in the rotating direction, which further complicated the control.
In the constitution in which the first and second moving frames follow the lens unit, the load acting on the torque source of the optical drive unit increases, and the entire apparatus becomes not only larger in size, but also higher on cost.
Therefore, in the light of such problems, it gives rise to the necessity of power transmission mechanism of optical system which can be simplified in control, and is inexpensive and lightweight.